Unfortunately there is no provision for that.
The code creates graphics objects with Tag 'head' (line object), 'tail' (animated line), and 'body' (animated line), but does not have any option to pass in or control sizes, and it runs the complete animation loop before returning.
What you can do is,
set(groot,'DefaultAnimatedLineLineWidth', SIZE)
and then call comet(), and then set it back again afterwards.
In theory you could set the axes default instead of groot default, but you probably need to "hold on" the axes for that to work.
